Various calorimetric flow transducers have been proposed previously. One type is represented by a hot-wire anemometer, in which a thin wire is heated by passage of an electric current, while its electrical resistance is sensed. When the wire is placed within a flowing fluid, typically a gas, a cooling effect of the fluid changes resistivity of the wire, which resistivity provides a measure of velocity of the flowing fluid. Variations of this principle include thin-film and semiconductor implementations.
In another type of calorimetric flow transducer, thermal energy is injected into a fluid at a constant rate, typically by means of an electrical heater. Fluid flowing near the heating element is raised in temperature, and the rise in temperature of the fluid is detected by an electrical sensor (refer U.S. Pat. Nos. 4,028,689, 4,501,144 and 6,314,807 and WO91/19170). This differs from the hot-wire anemometer in that a temperature-rise of the fluid, rather than the cooling effect of the fluid, is detected. The temperature-rise may be sensed downstream from the heater, using a thermistor for example. Improved versions of this principle utilize two temperature-sensors, one upstream of the heater, or co-located with it, and one downstream or spaced some distance from it. A comparison or differential measurement is made between the two temperature-sensors. This arrangement makes the transducer less susceptible to variations in ambient temperature of the associated fluid.
In a transit-time calorimetric flow transducer, heat is applied in the form of a thermal pulse, typically from an electrical heating element driven by an electrical pulse (refer U.S. Pat. Nos. 4,458,709 and 6,289,746). A bolus of heated fluid is carried downstream as part of overall flow. At some time subsequent to the applied pulse, a downstream sensor detects a temperature increase above the prevailing, or ambient, fluid temperature. After a further time-delay, the downstream sensor detects a fall in temperature, when most of the heated fluid has passed. These events may be called the leading and falling edges of the temperature pulse. Generally, there is some mixing of the heated fluid with unheated fluid, so that the thermal pulse passing the downstream sensor is spread out in time, or dispersed, compared with the upstream applied pulse. The transit-time transducer has a disadvantage of requiring access to the fluid stream in two places: a site where thermal energy is injected, and a downstream site for temperature-sensing.